SV: [Icc-avr] Are 'local' vars in main automatuc or in bss?

Fri Oct 26 02:26:07 PDT 2007

David, can you show us in an example how it should be done? I try to do just
as you describe but I often step into problems with error like double
defines and so on. 
I have tried using the typical #ifdef MAIN and so on but have failed making
it work properly. I usually have a header file for every c file (that should
be the very basics) and in that header file I put declarations on the
functions. The header file is included in every other file that uses the
functions. I think one reason for my problems may be that I usually in the
"project.h" (there my main is) have a lot of constants and maybe some
typedef that I want the other files be aware of so I include it in all other
project files (not reused files like drivers). The compiler do not like that
I put extern everywhere, as it is used even in my project.c. I do not want
to explicity write in every global variable in the c-files (extern nn), I
want to just include a .h file, I hope that is the normal procedure.

You can expand my little example in any way you like. I tried to use
self-descriptional names. There should I put the extern and there should I
put the typedef customType?

/Bengt

To be placed somewhere:
typedef customType...
#define A_VALUE 1

Project.h
=========
??? what should be in here?

Project.c
=========
static char projectFileGlobalVar;
char globalVar1;

customType globalCustomVar;

void AFunctionInProject(void)
{
}

void main(void)
{
  char projectFunctionLocalVar;

  projectFunctionLocalVar = A_VALUE;
  projectFileGlobalVar = A_VALUE;
  globalVar1 = A_VALUE;
  globalVar2 = A_VALUE;
  globalCustomVar = A_VALUE;
}

Module.h
========
??? what should be in here?

Module.c
========
static char moduleFileGlobalVar;
char globalVar2;

void AFunction(void)
{
  Char moduleFunctionLocalVar;

  moduleFunctionLocalVar = A_VALUE;
  moduleFileGlobalVar = A_VALUE;
  globalVar1 = A_VALUE;
  globalVar2 = A_VALUE;
  globalCustomVar = A_VALUE;
}

> -----Ursprungligt meddelande-----
> Från: icc-avr-bounces at imagecraft.com [mailto:icc-avr-
> bounces at imagecraft.com] För David Brown
> Skickat: den 26 oktober 2007 10:28
> Till: Discussion list for ICCAVR and ICCtiny Users. You do NOT need
> tosubscribe to icc-announce if you are a member of this.
> Ämne: Re: [Icc-avr] Are 'local' vars in main automatuc or in bss?
> 
> Graham Whyte wrote:
> > main() is no different from any other subroutine except that it gets
> > called at startup
> >
> > Variables declared outside subroutines or functions in any module file
> > are Global within that module
> > The can be referenced from anywhere within that module
> 
> Correct up to here...
> 
> > To make them Global to other modules, each other module needs to declare
> > then as External
> 
> That's wrong, and very misleading.
> 
> Variables (and functions) get global linkage if they are defined at file
> scope without "static" - C defaults to making them global and externally
> accessible.  Thus two "int x;" definitions at file scope in two modules
> refer to the same variable at the same address, regardless of any
> "extern" declarations.
> 
> The point of "extern" is to tell the compiler about a global variable
> (or function) without defining it - it allows access to a global
> variable, but does not turn a variable into a global one.
> 
> > This is exactly the same as making a subroutine available to other
> modules
> > The declarations must of course be identical
> >
> 
> The declaration *should* be identical - but be careful, because C allows
> you to get it completely wrong and still be legal C.
> 
> > You can make this very neat and tidy by putting the Globals in a
> > separate module
> > Then create the external  declarations in a separate header file which
> > you can #include wherever you need
> > You then only got one copy to worry about
> 
> I've seen programs made like this - as Ton says, "Yuck!".
> 
> If you want to write programs that are readable, understandable,
> debugable and maintainable, and allow for at least some level of re-use
> between programs, then you will avoid this setup like the plague.
> 
> Modularisation is critical to writing software.  Break your program into
> parts that fit logically together, and figure out what data and
> functions (and types, macros, etc.) that module makes available to other
> parts of the whole program.  This information is your module's
> interface, or API, and forms its "module.h" header file.
> 
> The header file "module.h" contains comments about how to use the
> module, along with all type declarations, #define's, "extern"
> declarations of data and functions, and any #include's that are needed
> to *use* the module (for example, if it contains a line "extern uint8_t
> count;", then it should also contain a line "#include <stdint.h>" or
> equivalent).
> 
> The C file "module.c" contains the *implementation* of the module, and
> has #include "module." near the beginning.  Every file-scope variable or
> function is either private to the module, and defined "static", *or* it
> has a corresponding "extern" declaration in "module.h".  There should be
> no exceptions (except perhaps temporarily to aid debugging, since you
> can see non-static items in the map file).
> 
> 
> As I said in an earlier post, I tend to rant a bit and treat my methods
> as the only correct methods, but in this case there is no real doubt.
> If you look at any other properly structured programming language, such
> as Pascal, Modula 2, or Ada, you'll see that this is exactly the way
> good modular programming is always done - other languages enforce these
> rules.
> 
> mvh.,
> 
> David
> 
> 
> >
> > Graham
> >
> > -----Original Message-----
> > *From:* icc-avr-bounces at imagecraft.com
> > [mailto:icc-avr-bounces at imagecraft.com]*On Behalf Of *bobgardner at aol.com
> > *Sent:* 25 October 2007 14:30
> > *To:* icc-avr at imagecraft.com
> > *Subject:* [Icc-avr] Are 'local' vars in main automatuc or in bss?
> >
> > Great summary of 'static' vars.... I remember in pascal the scope
> > 'nested' and stuff 'up a level' was visible, but c doesnt allow nested
> > functions I guess. Just to clarify, if I declare vars in main, are they
> > local or in the bss? Is main a subroutine that is called with params and
> > returns a value? Is the iccavr microcontroller model different from a
> > ram based system with an os?
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